1. Field of Invention
The present invention relates to a surface light source device of side light type, and more particularly to a technique for improving uniformity of output light in the surface light source device. The present invention is applied, for example, to backlighting for a liquid crystal display.
2. Related Art
A surface light source device of side light type is conventionally applied to a liquid crystal display to illuminate a liquid crystal panel from its back surface. The arrangement is suitable for making an entire shape of the surface light source device thin.
A bar-shaped light source such as a cold cathode tube, etc. is generally adopted as a primary light source in the surface light source device of side light type and is arranged on the side of a guide plate (a guide body formed in a plate shape). Illuminating light emitted from the primary light source is introduced into the guide plate through a side end surface of the guide plate. The introduced illuminating light is propagated within the guide plate and light is emitted from a major surface of the guide plate toward a liquid crystal panel during the propagation.
It is known that the guide plate adopted in such a surface light source device of side light type is of a type having a substantially uniform plate thickness and a type having a tendency in which a plate thickness is reduced as distance between the guide plate and the primary light source is increased. In general, the latter efficiently emits illuminating light in comparison with the former.
FIG. 8 is an exploded perspective view showing a surface light source device of side light type using the guide plate of the latter type. FIG. 9 shows a cross section taken along line Axe2x80x94A of FIG. 8. With reference to FIGS. 8 and 9, the surface light source device 1 of side light type has a guide plate 2, a primary light source 3, a reflection sheet 4, a light diffusive sheet H and prism sheets 5, 6 serving as light control members. The primary light source 3 is made of a light source element 7 and a reflector 8. The elements except for the guide plate 2 and the primary light source element 7 are also called xe2x80x9cadditive membersxe2x80x9d. The additive members illustrated in FIGS. 8 and 9 are the reflection sheet 4, the light diffusive sheet H, the prism sheets 5, 6 and the reflector 8.
The guide plate 2 is a transparent guide plate having a wedge-shaped cross section. For example, acrylic resin is adopted as a material of the guide plate 2. The guide plate 2 has two major surfaces 2B, 2C. One major surface 2C is employed to emit illuminating light and is called an emission surface. The other major surface 2B is called a back surface. For example, a light diffusive surface is formed on the back surface 2B by mat processing in a normal example.
The light source element 7 is, for example, a cold cathode tube (fluorescent lamp) and a reflector 8 having a nearly semicircular shape in cross section is arranged behind the light source element 7. Illuminating light is supplied to a side end surface 2 of the guide plate 2 through an opening of the reflector 8. A sheet-like regular reflection member made of a metallic foil, etc., or a sheet-like irregular reflection member made of a white PET film, etc. is adopted as the reflection sheet 4.
Illuminating light L from the primary light source 3 is introduced into the guide plate 2 through the incidence surface 2A and is then propagated toward a distal end of the guide plate 2 while the illuminating light is repeatedly reflected between the back surface 2B and the emission surface 2C. The reflection sheet 4 is arranged along the back surface 2B.
Incident angles of illuminating light to the emission surface 2C are reduced stepwise every reflection on the slope 2B. The reduction causes increase in light components having angle equal to or smaller than the critical angle with respect to the emission surface, and promotes light emission from the emission surface. Thus, deficiency of emission in an area far from the primary light source 3 is prevented.
Illuminating light emitted from the emission surface 2C has a property of scattering light since the illuminating light is further irregularly reflected on the back surface 2B having a light diffusive property, or the reflection sheet 4. However, a main propagating direction of illuminating light emitted from the guide plate 2 is inclined to a distal direction (a direction opposed to the primary light source 3) with respect to a frontal direction. Namely, the emitted light of the guide plate 2 has directivity. Such property of the guide plate is called directive emitting property.
The light diffusive sheet H is arranged to diffuse illuminating light emitted from the guide plate 2 and prevent a light diffusive surface of the back surface 2B from being visually recognized from above the emission surface 2C. Further, the light diffusive sheet H is arranged to prevent conspicuous glow and shade portions from appearing in the guide plate 2 lightened by illuminating light.
The prism sheets 5, 6 are arranged to correct emitting directivity of the guide plate 2. For example, the prism sheets 5, 6 are formed by a transmissive sheet material such as polycarbonate. In many cases, the prism sheets 5, 6 are arranged so that a prism surface of the prism sheet 5 is opposite with the guide plate 2 and a prism surface of the prism sheet 6 also is opposite with the guide plate 2.
Each of the prism surfaces has many projections having a triangular shape in cross section and extending in an approximately parallel direction. The inside prism sheet 5 is orientated so that its projections run in parallel with the incidence surface 2A. The outside prism sheet 6 is orientated so that its projections run in a direction approximately perpendicular to the incidence surface 2A.
Slopes of the projections correct the main emitting direction of the emitted light to a frontal direction of the emission surface 2C. A so-called double-sided prism sheet in which prism surfaces are formed on its both sides may be used.
In general, a surface light source devices of side light type adopting such a wedge-shaped guide plate and the prism sheets emit light in the frontal direction more efficiently than a surface light source device of side light type adopting a guide plate having a substantially uniform thickness.
However, an undesirable bright line appears on the guide plate 2 in the conventional device explained above so that uniformity of output light is reduced. As illustrated by reference sign K in FIG. 8, one to several bright lines are generated as a local thin high luminance band on the emission surface 2C near the incidence surface 2A. If a very high light diffusive property is given to the light diffusive sheet H, such bright lines are inconspicuous, but an entire reduction in luminance is caused.
An object of the present invention is to provide a surface light source device of side light type with improved uniformity of output light by preventing bright lines from being generated on a guide plate.
The present invention is applied to a surface light source device of side light type comprising a guide plate having major surfaces for providing an emission surface and a back surface, a primary light source for supplying illuminating light through an incidence surface of a side portion of the guide plate, first and second edges at which the major surfaces respectively intersect the incidence surface, and one or plural additive members arranged in direct contact with the emission surface and/or the back surface thereon in the vicinity of the first edge and/or the second edge.
In accordance with features of the present invention, at least one of the additive members has a shield for preventing illuminating light from being incident to the first or second edge.
The shield may include an absorptive area formed on the additive member(s) and each absorptive area may be formed so as to cross the first or second edge. The shield may be formed on an additive member arranged along the emission surface and/or the back surface.
Further, the additive member(s) include(s) a reflector arranged in the primary light source, and a distal end portion of the reflector may also provide a shield. In the case, a turn-up may be also formed in one or two distal end portions of the reflector so as to cross at least one of the first and second edges. A bend may be formed in one or two distal end portions of the reflector as locating just before the first or second edge. A J-curve portion may be formed in one or two distal end portions of the reflector as curving just before the first or second edge.
It is preferable that height of the reflector is greater than distance between the first and second edges in a position separated far from the emission surface, and is approximately equal to distance between the first and second edges in the vicinity of the incidence surface.
The additive member(s) include(s) a frame for surrounding the primary light source from its back, and a distal end portion of the frame may also provide a shield. A projecting portion may be projected inside and may form a slope in one or two distal end portions of the frame so as to block incidence of light to the first or second edge in order to provide a shield. It is preferable that height of the frame is greater than distance between the first and second edges in a position separated far from the emission surface, and is approximately equal to distance between the first and second edges in the vicinity of the incidence surface.
Blocking of light incidence to the edges prevents generation of luminance irregularities K (see FIG. 8 and related explanation) in the from of a bright line which would be generated in the prior art, even when the present invention is embodied in any one of the embodiments.
The present invention will next be described further in detail with reference to the accompanying drawings.